If you want to reset their inventories, ender chest, etc. data, do the following:
-> Go to Settings tab and note down your Level Name.
-> Now Stop your server completely.
-> Go to File Manager tab.
-> Look for the folder which has the Level Name which you noted down earlier.
-> Open this folder, and delete playerdata folder and make a new empty folder named playerdata.
-> Start your server.
This will reset all player data such as inventory, ender chest, last location, EXP value, active potion effects, armor, etc.